160-Meter Dipole Height Show N0GW - Gary Wescom - 1 Nov 2006 During the current lull in solar activity, operation on the 160 meter ham band has become more popular. In the evening when most of us have time to relax and enjoy a little time on the radio, the higher frequency bands are either dead or very crowded. 160 meters, in contrast is normally open for contacts out to several hundred miles away with little interference other than static from
thunderstorms. During the winter, when lightning storms are infrequent, 160 meters can be very enjoyable.
Table 1 – Dipole performance over average ground. Figure 1 – Radiation pattern for dipole at 50 feet. 2 Figure 1 shows a 3 dimensional plot of the radiation pattern of a dipole mounted at 50 feet above the ground. As can be seen, its shape is roughly that of a sphere that is compressed inward slightly at the ends of the dipole elements. Maximum radiation is straight up which is good for communications out to a few hundred miles. Some other antenna type with stronger low angle radiation would probably be better for DX operation. Restated, table 1 shows us that compared with a 160 meter dipole mounted at 100 feet above the ground, there would be essentially no detectable difference lowering it to 60 feet. Also, even down to 40 feet or so, the theoretical disadvantage is less than 3 dB. What’s more, the expected SWR on the transmission line at resonance is quite low. OK, now that was interesting but I’ve learned that sometimes pure theoretical dipole configurations do not to match well with more realistic antenna installations. In the case of 160 meter dipoles, a flat-top configuration is not often practical. It is difficult to find two tall supports at an appropriate distance apart that would survive the tension required to hold a 250 foot long dipole straight, especially with the weight of the its feedline hanging from the middle. Most of us use an Inverted-Vee configuration on this band. An Inverted-Vee configuration allows us to locate the high current center of the dipole high in the air while securing its ends to more convenient shorter supports. While still basically a dipole, experience and verification with past analysis showed that there are differences in how flat-top and Inverted-Vee configurations perform. I was curious if that would put the Inverted-Vee configuration at a disadvantage on 160 meters. Table 2 shows the results of my Inverted-Vee calculations. The problem I ran into in making those calculations was with the size of the antenna. An Inverted-Vee antenna is typically defined as having an included angle of 120 degrees or less, down to minimum of about 90 degrees. The dipole halves are so long that with a 120 degree included angle, the antenna feed point would have to be higher than 60 feet just to keep their ends from touching the ground. What I did to try to find a more reasonable set of antenna configurations to model was to simply keep the ends of the antenna at 15 feet above the ground. That was an arbitrary choice on my part but seemed to be reasonable and practical. The Vee angle then was determined simply by the geometry that resulted at each feed point height. In general, relatively little performance was lost when compared with the flat-top dipole. The gain numbers are somewhat lower at each feed point height due to greater ground losses from parts of the antenna being closer to the ground. Another reason the gain numbers are lower is that the radiation pattern is less directional. The flat-top dipole radiation pattern was not quite circular with a small loss of low angle radiation off the ends. As may be seen in figure 2, the Inverted-Vee radiation pattern is almost perfectly circular. 3 Figure 2 – Radiation pattern for Inverted-Vee at 50 feet. As with the flat-top configuration, the Inverted-Vee configuration showed no significant performance improvement above 60 feet. Performance is still within 3 dB down to below 40 feet. Notice also the SWR column. It seems the Inverted-Vee configuration with the ends 15 feet above the ground is great for having low SWR at resonance.
Table 2 – Inverted-Vee with ends at 15 feet above the ground What is the length of a 160 meter dipole?As stated in the introduction to this article, the first biggest challenge for operating on 160m is the physical size of an efficient transmitting antenna. The length of a half wavelength dipole at 1.85 MHz is approximately 253 feet (each side would be about 127 feet).
How long is a 160 meter end fed antenna?In fact, it looks more like an end-fed 1/4 wave 160 meter antenna (130 ft or 39.624 m long) which goes up some 25 feet (7-8 m), then over horizontally, at an average of 25 feet (7-8 m) above ground, to a tree at the far end of my property.
How long should my dipole antenna be?When determining how long to make each leg of a Dipole antenna, dividing the frequency in MHz into 468 will give the overall length. To find the half wave, divide that number by two. This will give you the length of each leg of the antenna.
Will an 80 meter dipole work on 160 meters?For use on 160, the 80 meter dipole becomes a T-shaped (top loaded) vertical radiator, fed against ground. The vertical section (the “feeders”, ladder line, window line, or twin lead for 80, 40 or 20) function as the principle radiator.
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